If you made money this year without an employer withholding taxes from it, the IRS expects a check four times a year.
That next check is due September 15, and it covers income you earned from June through August.
Miss it, and the penalty clock starts ticking immediately.
Here's the part that trips people up: there's no W-2 safety net.
If you drive for a rideshare app, sell on Etsy, tutor, freelance, or pull in rental income, you're responsible for both the income tax and the self-employment tax, which covers Social Security and Medicare.
That self-employment piece alone runs 15.3 percent on net earnings, on top of whatever you owe in regular income tax.
The IRS doesn't send a bill before the deadline.
It waits until you file, then adds interest and a penalty on whatever you underpaid.
The underpayment penalty currently runs at an annualized rate that changes each quarter, and it compounds.
Waiting until April to square up means paying extra for money you could have set aside months ago.
The good news is the system is more forgiving than its reputation.
You generally avoid penalties if you pay at least 90 percent of this year's tax bill or 100 percent of last year's, whichever is smaller.
If your income jumped this year, that last-year safe harbor is often the easier target.
If you earned more than $150,000 last year, the safe harbor rises to 110 percent of your prior-year liability.
The simplest way to stay current is to pay yourself like an employer would.
Every time a client payment lands, move 25 to 30 percent into a separate savings account.
Then send the IRS its cut from that account on the quarterly dates: April 15, June 15, September 15, and January 15.
You can pay free through IRS Direct Pay or your IRS online account, and the money leaves your checking account without a processing fee.
If your income is steady and you'd rather not think about deadlines, you can ask the IRS to set up an automatic payment plan, or you can have a spouse's paycheck withholding increased to cover the gap.
That second option is a legitimate strategy, not a loophole.
Withholding is treated as paid evenly throughout the year, so a bigger paycheck deduction in December can erase an earlier shortfall.
One more thing worth checking: if you also have a regular job, your employer's withholding might already cover your side hustle.
The IRS withholding estimator at irs.gov walks you through it in about ten minutes.
Run it now, while there's still time to adjust, rather than in March when your options are gone.
The people who get hurt by quarterly taxes aren't the ones with complicated finances.
They're the ones who didn't know the rules existed until a penalty showed up.
Final Thoughts
Set aside the percentage, mark the four dates, and the whole thing becomes boring, which is exactly what you want from the IRS.